call_rcu: handle retry without wait correctly

The wait scheme has an implementation problem: if the list is not empty
when the !RT scheme checks for it, it will restart the loop and
decrement the futex (again) without calling call_rcu_wait() (which would
wait until it is set back to 0). So in this case, we can end up
decrementing "futex" to values well below -1.

Fix this by moving the decrement before the loop, and duplicate it after
return from call_rcu_wait() + poll() delay. Also move the "set futex to
0 upon stopping" outside of the loop: this is the only way the loop can
be stopped anyway.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>